Refactor NameID format related tests

**Why**:
1. The tests are hard to understand and do not clearly show the
   different contexts
2. The tests assert on a side effect (the analytics event posted) rather
   than the actual desired behavior (the return values)
3. The tests are actually misleading - they conflate "unspecified" and
   unsupported

**How**:
- Refactor tests with clear contexts and shared examples
- Assert on actual return value and not on a side effect
- Clean up test case descriptions
- Assert that the correct attributes are logged in the analytics event

changelog: Internal, Automated Testing, Refactor NameID format related tests

lib/saml_idp_constants.rb

@@ -30,6 +30,7 @@ module Constants
 
       NAME_ID_FORMAT_PERSISTENT = 'urn:oasis:names:tc:SAML:2.0:nameid-format:persistent'
       NAME_ID_FORMAT_EMAIL = 'urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress'
+      NAME_ID_FORMAT_UNSPECIFIED = 'urn:oasis:names:tc:SAML:1.1:nameid-format:unspecified'
       VALID_NAME_ID_FORMATS = [NAME_ID_FORMAT_PERSISTENT, NAME_ID_FORMAT_EMAIL].freeze
 
       REQUESTED_ATTRIBUTES_CLASSREF = 'http://idmanagement.gov/ns/requested_attributes?ReqAttr='